One moment, manager Joe Maddon was pointing out how “spectacular” and “invaluable” Mike Montgomery has been as a starter for the Cubs this season.

The next, Maddon was revealing that Montgomery would pitch out of the bullpen this weekend against the Nationals.

The Cubs also pushed Montgomery’s next scheduled start to Aug. 18. Maddon said he wants to “be proactive with Mike. We want to monitor his time out there a little bit so that he can be effective into September.”

Montgomery, who has been the Cubs’ most consistent starter of late, has lasted at least five innings in each of his 13 starts. The left-hander began the season in the bullpen but has made no secret that he prefers to start. His performance has proven he can be a force in the rotation.

“There is no longer a motivation of just trying to prove myself — I feel like I have,” Montgomery said before the Cubs defeated the Nationals 3-2 at Wrigley Field. “It’s just about playing an important role on this team. If being in the bullpen is more valuable, then I’ll do that.

“Coming down the stretch and in the playoffs, the value of your bullpen is important. I just want to put myself in a position to pitch meaningful innings. It could be starting or relieving.”

Montgomery had been scheduled to start against the Brewers on Tuesday at Wrigley Field, but Jose Quintana will face the Central Division rivals instead. He’ll be followed by Kyle Hendricks on Wednesday.

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“If you feel good, it shouldn’t matter too much when you’re pitching,” Montgomery said. “And I feel pretty good right now.”

New guy

The Cubs signed veteran reliever Jorge De La Rosa to a major-league deal, and the left-hander was available to pitch Friday.

“We’re just trying to augment that left-handed side coming out of the bullpen,” Maddon said. “[De La Rosa has] plenty of experience. That creates more depth.”

The 37-year-old was 0-2 with six holds and a 4.63 ERA in 42 appearances with the Diamondbacks before being released Aug. 4.

“Everybody knows this is one of the best teams in the division,” De La Rosa said of the Cubs. “I’m excited to be here with these great players, and I’ll just try to do the things to help any way I can here.”

Maddon said De La Rosa will be used primarily against left-handed hitters. He has limited lefties to a .220 average this season.

“I’m ready for everything — I can do anything,” De La Rosa said. “I was pitching in any situation in Arizona, and I think it will be the same thing here.”

To make room for De La Rosa on the 25-man roster, the Cubs optioned Randy Rosario to Class AAA Iowa. Rosario is 4-0 with seven holds and a 3.00 ERA in 28 appearances.

In a procedural move that had nothing to do with his recovery from tendinitis in his right triceps, starter Yu Darvish was transferred to the 60-day disabled list.